This invention relates to epoxy-functional organopolysiloxanes. More particularly, this invention relates to pre-crosslinked epoxy-functional organopolysiloxanes and their use as conformal coatings.
UV-curable silicone compositions have become widely accepted as protective coatings for electronic components mounted on circuit boards. The moisture resistance, thermal stability and resistivity of silicones make them ideal for this purpose. The cure chemistry of radiation curable silicones is most often free-radical in nature, requiring a high flux of radicals generated by UV light radiation of photoinitiators.
A drawback to free-radical crosslinking processes is that such processes are subject to inhibition by atmospheric oxygen. The "oxygen effect" is particularly troublesome in oxygen-permeable silicone systems. Prompt cure response, therefore, is dependent on either efficient nitrogen blanketing to forestall oxygen cure inhibition or the use of amine-benzophenone-type synergist catalyst systems to overcome the oxygen effect.
Production ultraviolet facilities are not easily nor economically inerted, particularly if wide web converting or high speed processing is practiced. Furthermore, amine-benzophenone-type synergist catalyst systems useful for overcoming oxygen inhibition in organic acrylate UV cure resins are often insoluble in dimethyl silicone polymers. Fast non-inerted UV cure is achievable by combining mercapto- and acrylated-silicones with certain photosensitizers, but such systems are meta-stable and subject to shelf-life and pot-life limitations.
Currently, UV-curable silicone coatings useful for conformal coatings, optical fiber coatings, and electrical encapsulation contain silicon-bonded mercapto-olefin or acrylate radicals. Reference is made, for example, to U.S. Pat. Nos. 4,558,082; 4,585,669; 4,587,137; 4,496,210; and 4,780,486.
However, the use of mercapto-olefins or acrylates has several drawbacks. For example, acrylates are toxic and both acrylate monomers and mercaptans give off highly offensive odors which can persist in the cured products, and because of precautionary/safety measures entailed by their use, they are not readily adapted to industrial applications.
It is desirable therefore to provide conformal coatings and coatings for optical fibers and electrical encapsulation which are made from UV-curable silicone compositions which do not contain mercapto-olefin or acrylate functionality and which do not cure in a free-radical based crosslinking process.
Cationic (non-radical) UV cure processes are not affected by oxygen and are therefore well suited for high speed silicone coating and curing processes. Cationic UV cure of silicone coatings has been found to be most practical with epoxy-functional silicones.
It would be desirable, therefore, to provide conformal coatings and coatings for optical fibers and electrical encapsulation which are made from UV-curable epoxy-functional silicone compositions.
Epoxysilicone compositions containing epoxy-functionalized silicone polymers and silicone miscible iodonium photocatalysts are presently used as release coatings.
High speed solventless silicone release processing requirements dictate that UV-curable epoxysilicone release compositions be limited to unfilled, low molecular weight, highly organofunctionalized polymers whose cured films are therefore brittle, easily abraded, weak substances unsuited for other coating applications.
Conformal coatings are desired which are tough and have high tensile strength and high percents of elongation but which nonetheless may be UV-cured very rapidly.
Accordingly, it was desirable to provide UV-curable epoxy-functional silicone coatings with improved film properties for use as conformal coatings and coatings for optical fibers and electrical encapsulation.
It was further desirable to provide epoxy-functional silicone coatings with improved film properties without using fillers. Fillers tend to block UV light which consequently slows down cure dramatically. Furthermore, it is difficult to incorporate fillers into low viscosity matrices such as epoxy-functional silicone polymers.